1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an internal combustion engine.
2. Description of the Related Art
The exhaust gas of a diesel engine contains particulates, comprising carbon as the chief component, and NOX. It is desired that the particulates and NOX are not emitted into the atmosphere. For this reason, it has been suggested that a device for purifying the particulates and a device for purifying NOX are integrally or separately arranged in the exhaust system.
The device for purifying the particulates is one which traps the particulates. If the particulates trapped on the device are not removed, resistance to the exhaust gas therein will increase greatly. To oxidize and remove the trapped particulates, it may be required that a reducing material such as fuel is supplied to the device for purifying the particulates and to raise the temperature thereof. On the other hand, to allow the absorbed NOX to be released for reducing and purifying in the device for purifying NOX, it is required that a reducing material such as fuel is supplied thereto to make the air-fuel ratio in the device be desirably stoichiometric or rich. Thus, a reducing material supply device may be provided in the exhaust system in which the particulates and NOX can be purified, as disclosed in Japanese Unexamined Patent Publication No. 2001-317338.
Of course, the reducing material supply device arranged in the exhaust system can control an amount of reducing material per unit time. By the way, the amount of reducing material per the unit time required to oxidize and remove the trapped particulates in the device for purifying the particulates is greatly different from that required to release and reduce NOX in the device for purifying NOX, and they also change in accordance with an amount of exhaust gas. Namely, the reducing material supply means arranged in the exhaust system must control an amount of supplied reducing material per the unit time over a wide range and thus requires a very large range of control of the amount of supplied reducing material.
However, one reducing material supply device cannot realize such a large range of control the amount of supplied reducing material. Accordingly, even if the device can precisely control the amount of supplied reducing material in either the case that an amount of supplied reducing material per the unit time is large or in the case that it is small, it cannot precisely control the amount of supplied reducing material in both cases. The above related art also discloses that some reducing material supply devices are arranged in the exhaust system. However, even if some reducing material supply devices are arranged in the exhaust system, the required large range of control of the amount of supplied reducing material cannot be realized.